Our objective is to elucidate the hereditary control of organelle structure and function by studying mutations affecting chloroplasts and mitochondria. We are especially interested in defining the roles played by the chloroplast and mitochondrial genomes in the biogenesis of these organelles and in mapping these organelle genomes by a combination of genetic and physical methods. The unicellular green alga Chlamydomonas reinhardtii is a model system which allows isolation and characterization of nonphotosynthetic mutants lacking many different chloroplast components (acetate requirers) and mutants defective in specific mitochondrial functions (obligate photoautotrophs). In addition, antibiotic resistant mutations affecting organelle ribosomes and electron transport in the chloroplast and mitochondria can be readily obtained. In no other eukaryotic cell at present can mutations of both the chloroplast and mitochondria be investigated to determine the genetic interrelationship of these organelles to the nucleus and to each other. We are also investigating in detail the role of chloroplast and nuclear genes in chloroplast ribosome biogenesis. Not only is the chloroplast and nuclear genes in chloroplast ribosome biogenesis. Not only is the chloroplast ribosome of C. reinhardtii particularly well adapted for the study of the genetic control of organelle ribosome formation, but the results we obtain may well give us insight into the organization of the nuclear genes involved in eukaryotic ribosome biogenesis generally.